Switched mode power supplies as key components of telecom and commercial systems often dictate their size and electrical performance as well as reliability and costs. As requirements for the key characteristics of power converters (e.g. power density and efficiency) increase, the demands of these key characteristics increase for inductive components in particular. One approach of increasing the power density and the efficiency in such systems is to integrate the inductive components. For example, transformers and inductors can be integrated into a single magnetic structure, thereby reducing cost, increasing power density and equally increasing power efficiency.
Circuits where integrated magnetics are highly advantageous are soft switching converters (U.S. Pat. No. 6,862,195), which are capable of yielding high efficiency while operating at high switching frequency. A typical soft switching converter uses three magnetic components: a parallel resonant inductor, a two-winding transformer and a series filter inductor. This converter results, additionally to the number of discrete magnetic components which yield higher size and costs, in at least three windings and several interconnections which negatively impact the efficiency.
The parallel primary resonant inductor and the transformer are generally integrated into one component. An air gap is ground in the non ideal transformer in order to adjust the magnetizing inductance which replaces the parallel primary resonant inductance.
In recent years some efforts were done to integrate all three magnetic components into a single component for LLC resonant converter. Some integrated magnetic structures are shown in US 2008/0224809. An additional inductor winding, which constitutes the series resonant inductance, is introduced to enhance the leakage inductance of the transformer.
While US 2008/0224809 addresses some appropriate ameliorations to typical soft resonant LLC converters, there are still some setbacks: mostly E-cores from retail are employed and bobbins are unavoidable to wind the coils. The bobbins negatively affect the costs, the power density, the power efficiency and the thermal distribution. There are supplementary power losses due to air gap fringing fields and higher winding mean length. The bobbins are costly and cause more leakage and inductance losses. Additionally, they reduce the power density and increase the thermal resistance. Furthermore, the integrated magnetic components disclosed in US 2008/0224809 are comparably complicated to manufacture due to their complex geometries, their number of windings and the relative positions of these windings in respect to each other.